Microwave hyperthermia probe

ABSTRACT

Heating pattern uniformity is provided in a coaxial microwave hyperthermia probe by varying the open area in the outer conductor in an axial direction such that there is a maximum open area in the axial center portion. The variations may be provided by winding an outer conductor in a helical pattern with a variable pitch or by cutting openings of axially varying size in a solid outer conductor. The invention is applicable to both flexible and rigid probes.

This is a division of co-pending U.S. application Ser. No. 07/547,535filed Jun. 29, 1990, now U.S. Pat. No. 5,097,845, which in turn is adivisional of now abandoned U.S. application Ser. No. 07/371,202 filedJun. 26, 1989, now abandoned, which in turn is a division of U.S.application Ser. No. 07/108,923 filed Oct. 15, 1987 by Richard W.Fetter, et al. now U.S. Pat. No. 4,841,988.

BACKGROUND OF THE INVENTION

The subject invention relates to microwave probes useful in hyperthermiatherapy, and more particularly, to probes utilizing a coaxial antennaconstruction and useful primarily for the interstitial or invasivehyperthermia treatment of tumors.

It has long been known that certain cancer cells can be destroyed atelevated temperatures which are slightly lower than temperaturesnormally injurious to healthy cells. In recent years, hyperthermiatherapy utilizing electromagnetic radiation has been found to beparticularly effective and many processes and types of apparatusutilizing microwave hyperthermia are known in the art. These includenon-invasive types in which external applicators are utilized and themicrowave energy is allowed to penetrate the skin and underlying tissue,including a tumor to be treated. Obviously, this results in the heatingof healthy tissue as well and this lack of control is one reason whymore direct and exact means of applying microwave hyperthermia have beensought.

Thus, invasive methods and related apparatus have been developed inwhich a hyperthermia probe or probes may be inserted to the point oftreatment via a normal body opening or may be inserted interstitiallythrough the skin directly to the site of the tumor to be treated. Suchinvasive methods and apparatus provide the advantage of potentiallybetter control of temperature of the mass or volume of tissue to betreated.

Since many types of malignancies cannot be reached and effectivelytreated with non-invasive techniques or with invasive probes designed tobe inserted into a normal body opening, much attention has been recentlygiven to long, narrow needle-like probes which can be inserted directlyinto the body tissue and to the site of the tumor or malignancy to betreated. Such probes must of necessity be of a very small diameter, bothto aid the ease with which they may be inserted and used and to reducethe trauma associated therewith to the patient.

Interstitial microwave hyperthermia probes may be of a rigid orsemi-rigid type with a needle-like point for direct insertion into thebody tissue. Alternately, the probe may be more flexible and adapted tobe used inside a catheter first inserted into the body tissue byordinary, well-known methods. The advantages of using a probe insertedinto a catheter include avoiding the need to separately sterilize theprobe and to take advantage of catheters which may already have beeninserted for other types of concurrent treatment, such as radiationtherapy. Nevertheless, the convenience of more rigid probes adapted fordirect interstitial insertion allow their alternative use in certainsituations.

Regardless of the type of microwave probe, a problem common to all ofthem has been to provide a uniform pattern of radiated energy andheating axially along and radially around the effective length of theprobe or to otherwise control and direct the heating pattern. A knownand predictable heating pattern is, of course, important so that theheating may be confined to the greatest extent possible to the tissue tobe treated and excessive heating of healthy tissue avoided.

Microwave probes of two kinds have been used, one comprising a monopolemicrowave antenna and the other a dipole coaxial antenna. In a monopoleantenna probe, a single elongated microwave conductor is exposed at theend of the probe (sometimes surrounded by a dielectric sleeve) and themicrowave energy radiates generally perpendicularly from the axis of theconductor. However, so-called monopole probes have been found to producenon-uniform and often unpredictable heating patterns and the heatingpattern does not extend beyond the probe tip. As a result, more recentattention has been directed to so-called "dipole" antennas of a coaxialconstruction. These include constructions having an external reentrantcoaxial "skirt" around the distal end of the outer conductor.

The typical coaxial probe includes a long, thin inner conductorextending along the axis of the probe, surrounded by a dielectricmaterial, and an outer conductor surrounding the dielectric. To providethe effective outward radiation of energy or heating, a portion orportions of the outer conductor can be selectively removed. This type ofconstruction is sometimes referred to as a "leaky waveguide" or "leakycoaxial" antenna. Obviously, variations in the location, size and areaof the outer conductive material removed along the effective length ofthe probe can significantly affect the heating pattern provided. One ofthe primary goals in such construction has, thus, been to provide auniform heating pattern generally or a more narrowly controlled anddirected pattern in a selected region of the probe tip. U.S. Pat. No.4,204,549 discloses the removal of short semi-cylindrical sections ofthe outer conductor in a coaxial probe to provide directional control ofthe heating pattern, but uniformity in the control of the heatingpattern is not discussed. U.S. Pat. No. 4,669,475 discloses a coaxialantenna probe in which a full circumferential cylindrical portion of theouter conductor is removed over a selected intermediate axial length ofthe probe. However, the heating pattern per se is not disclosed ordescribed, and the heating pattern of individual or multiply orientedprobes is controlled by varying the microwave energy supplied to theprobes. U.S. Pat. No. 4,658,836 discloses the removal of long axialsegments of the outer conductor to provide full length heating. Controlof the heating pattern is attained by varying the thickness ofsupplemental outer dielectric covering or by a unidirectional variationin the axial outer conductor segments. In an alternate embodiment, theouter conductor is attached in a uniform double spiral pattern toprovide the necessary open space for radiation leakage. However, theutility of the spiral pattern is disclosed as providing the probe withflexibility and to provide relative rotation of the heating patternalong the probe length. Finally, the probe of this patent is intendedparticularly for insertion into a body passage or cavity and not directinterstitial insertion into body tissue.

Control of the heating pattern in coaxial probes for interstitialhyperthermia treatment continues to be a problem. It is important to beable to control and predict the heating pattern axially along theeffective length of the probe. Because of the need to maintain the verysmall diameter of these probes, it is impractical to utilize heatingpattern control means which increase the effective diameter, such as anouter dielectric layer. Thus, any improved means of heating patterncontrol should be compatible with the typical constructions ofinterstitial probes, whether they be of the flexible or rigid type.

SUMMARY OF THE INVENTION

In accordance with the present invention, heating pattern uniformity isprovided in a coaxial microwave probe, both radially and axially alongits effective length, by varying the open area in the outer conductoraxially along the effective length of the probe such that there is amaximum open area in the axial center portion and smaller open areas inboth opposite directions.

In a preferred embodiment, the typical continuous outer conductor isremoved or eliminated along substantially the full effective heatinglength of the probe and replaced with a spiral winding of a conductivewire in which the winding has a varied spacing to provide the varyingopen area in the outer conductor. An extremely broad range of windingpatterns may be used depending on specific probe constructioncharacteristics. Typically, the effective length of the probe is dividedinto sections of different winding pitch. The sections may provide anoverall symmetrical pattern or be asymmetric, the pitch within a sectionmay be uniform or varying, transitioned pitch sections may be providedbetween the major sections, and each section itself may comprisesubsections with variations in the windings.

In an alternate embodiment particularly applicable to rigid orsemi-rigid probes in which the outer conductor is typically a thin solidmetal covering, the axially varying open area in the outer conductor isattained by providing a series of axially spaced slots of varying lengthand depth. The slots are preferably transversely disposed in axiallyspaced planes perpendicular to the axis of the probe. As with thespirally wound embodiment, the slots are sized or spaced to provide amaximum open area in the center portion of the effective length of theprobe and relatively smaller open areas axially in both directionstherefrom. The slot pattern may be symmetrical or asymmetric in an axialdirection, and may be otherwise varied in manner similar to the spirallywound embodiment.

The general arrangement of both embodiments has been found to providethe desired uniformity in the heating pattern, both radially and axiallyalong the probe. In both embodiments, heating extends beyond the tip.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal view, partly in section, of the embodiment ofthe invention particularly adaptable for use with flexible coaxialprobes.

FIG. 2 is a view similar to FIG. 1 showing a pattern in the wound outerconductor particularly suitable for longer probes.

FIG. 3 is an enlarged detail of a portion of the probe shown in FIG. 1.

FIG. 4 is a side view of a probe of a more rigid construction showing analternate embodiment of the invention.

FIG. 5 is a top view of the probe shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a microwave probe 10 is constructed from a sectionof conventional small diameter flexible coaxial cable. The cableincludes a wire-like center conductor which runs axially along thelength of the cable and is surrounded by a cylindrical layer of suitabledielectric material 12. Many types of dielectrics are usable and aplastic material, such as PTFE, is often used. The dielectric materialis, in turn, surrounded by an outer conductor 13 comprising a braidedmetal construction. Both the unitary center conductor 11 and the braidedwire outer conductor 13 may be made of a copper base metal which ispreferably silver-plated to enhance conductivity. Other suitable metalsmay, of course, also be used. The proximal end of the probe 10 includesa standard coaxial connector 14 for connection to a conventional sourceof microwave energy (not shown).

The conventional coaxial cable thus far described is modified to providethe present invention by removing a cylindrical section of the braidedouter conductor 13 along the distal end of the probe. The axial lengthof the section of the outer conductor removed determines the effectivelength of the probe from the standpoint of the length along which aneffective heating pattern may be produced. A short section of braidedouter conductor 16 may be left at the probe tip to facilitate theconductive connections to be hereinafter described, but that shortsection may be eliminated as well and the conductive connections made inanother manner.

In place of the section of the braided outer conductor 13 removed fromthe end of the cable, a thin wire 17 is wound in a helical pattern overthe dielectric 12 and between the outer conductor 13 at the proximal endof the probe to the free end thereof. Referring also to FIG. 3,appropriate conductive connections 18 and 19 are made between the wire17 and the ends of the outer conductor 13 between which the spiralwinding is made. Alternately, if the short section of outer conductor 16at the end of the probe is eliminated, the spiral winding is simply runto the probe tip.

In a specific embodiment of the probe shown in FIG. 1, which may have aneffective length (or axial heating pattern length) of about 4 cm(approximately 1.5 inches), the center conductor 11 has a diameter of0.007 inch. The cylindrical dielectric covering 12 has an outer diameterof 0.033 inch and the braided outer conductor is about 0.010 inch inthickness. The conductive wire winding 17 is 0.007 inches in diameter.Thus, the nominal OD of the probe along its effective length is 0.047inch.

The spiral winding pattern shown includes a center portion 20 and endportions 21 adjacent thereto. The wire 17 is preferably wound with aspaced pitch over the full length of the winding and with the maximumspacing in the center portion 20 to provide the maximum open area in theouter conductor for microwave energy leakage or radiation. In the probeshown, the center portion 20 has a length of about 0.250 inch and theend portions 21 lengths of 0.295 inch each. The spaced pitch of thewinding of wire 17 in the center portion 20 is 0.0281 inch and thespaced pitch of the end portions 21 is 0.0095 inch. Suitable pitchtransition zones may be provided between the portions 20 and 21.

The actual heating pattern provided by the probe extends axially in bothdirections beyond the conductive connection 18 and 19 between the wire17 and the outer conductor 13. Typically, the heating pattern willextend about 0.2 inch (0.5 cm) beyond the tip of the probe. Thus, theeffective heating pattern provided by the probe is approximately 4 cm.

FIG. 2 shows an alternate embodiment of the spirally wound probe shownin FIG. 1, adapted particularly for longer probe constructions. Overall,the components of the probe 23 in FIG. 2 are the same as those in theembodiment of FIG. 1 and are, therefore, identically numbered. Thus, aconventional coaxial cable has a conductive inner member 11, surroundedby a dielectric 12, and around which is disposed an outer conductor 13.The outer conductor, which may be of the braided construction previouslydescribed, is cut away to expose the dielectric material 12 at the endof the probe 23.

Beginning with a conductive connection 18 to the end of the outerconductor 13, a wire 12 is wound in a helical pattern which, in thisembodiment, runs to the end of the probe. The wire is wound in a spacedarrangement and the pitch of the winding varies over the length of theprobe with a maximum spaced pitch in the center portion. However, inthis embodiment, the center portion 24 of the winding itself comprises awinding having a variable spaced pitch.

In the specific construction shown, the center portion 24 of the windingincludes a narrowly spaced center subportion 27 and more widely spacedend subportions 28 either side. The end portions 26 again comprise awinding of a narrow spaced pitch. In one preferred construction, the endportions 26 each have a length of 0.472 inch and are wound to a pitch of0.0169 inch. The center end subportions 28 are each 0.689 inch in lengthand are wound to a pitch of 0.0276 inch each. The center subportion 27is identical to the end portions 26 having a length of 0.472 inch and apitch of 0.0169 inch.

The effective heating pattern of the FIG. 2 probe is approximately 8 cmor slightly greater than 3 inches. The heating pattern also has uniformradial depth along substantially its entire length. Depending on thepower level of the microwave energy supplied to the probe, the uniformheating pattern may extend radially for a centimeter or more. The axialextension of the heating pattern toward the proximal end of the probeover a short portion of the braided outer conductor 13 allows athermocouple to be located at that point. In this manner, thethermocouple or other heat sensor connections will not interfere withthe operation of the probe and yet are attached in an area where themeasured temperature is representative of that effectively applied bythe probe.

Referring now to FIGS. 4 and 5, there is shown an alternate embodimentof the probe which is of a rigid or semi-rigid construction and intendedfor direct insertion into body tissue without the use of a catheter. Theprobe 30 is of a basic coaxial construction, including an axial innerconductive member 31, a dielectric material 32 surrounding the innerconductor, and a conductive outer shell or layer 33. To provide thedesired pattern of heating to extend beyond the tip of the probe, theouter conductive layer 33 is suitably formed to a point and makesconductive contact 35 with the inner conductor 31 at the tip of theprobe.

To provide the axially varying open area in the outer conductive layer33, a series of transverse slots 34 is cut into the outer layer 33 andunderlying dielectric material 32 on diametrically opposite sides of theprobe 30. The slots 34 are of maximum length and depth in the centerportion of the axially disposed slot pattern and become progressivelysmaller in both axial directions therefrom. The deepest centrallylocated slot or slots may have a maximum depth of approximately 1/3 thediameter of the probe and the receding depths of the slots in oppositeaxial directions should preferably fit a smooth curve, as shown by thedashed line 36 in FIG. 5.

The rigid probe 30, in a preferred embodiment, has an outer diameter of0.086 inch. The slot pattern comprises nine slots 34 spaced 0.25 inchwith the small slot nearest the tip spaced 0.375 inch therefrom. Theforegoing dimensions, however, may be varied over fairly broad ranges.In order to provide a smooth outer surface on the probe and to bettermatch the probe impedance to the tissue in which it is intended to beused, the slots are filled with a material having a high dielectricconstant, such as titanium dioxide.

The dashed line 37 in FIG. 4 is representative of the uniform heatingpattern attained with each embodiment of the hyperthermia probe of thesubject invention. Each embodiment described hereinabove includes anaxially symmetrical pattern around the opening provided in the outerconductor. Such symmetry, though desirable, is not necessary andvariations including asymmetrical patterns in the openings may also beused to provide the desired heating pattern.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

We claim:
 1. In a coaxial microwave probe for performing benignprostatic hyperplasia treatment by inserting said probe into a normalbody opening and having a conductive inner member extending axiallyalong the length of the probe, a dielectric material disposed about theinner member and an outer conductive member disposed about thedielectric material, said outer conductive member at least in partdefining an effective heating length of the probe to provide a desiredheating pattern, the improvement comprising said outer conductive memberformed from a conductor wound in a helical pattern of axially varyingpitch.
 2. The invention as set forth in claim 1 wherein said helicalpattern comprises axially adjacent portions of different pitch.
 3. In acoaxial microwave probe for providing microwave treatment by selectivelyinserting said probe into a normal body opening of a patient orinserting said probe interstitially into said patient and having aconductive inner member extending axially along the length of the probe,a dielectric material disposed about the inner member and an outerconductive member disposed about the dielectric material, said outerconductive member at least in part defining an effective heating lengthof the probe to provide a desired heating pattern, the improvementcomprising said outer conductive member formed from a conductor wound ina helical pattern of axially varying pitch.
 4. The invention as setforth in claim 3 wherein said helical pattern comprises axially adjacentportions of different pitch.
 5. In a hyperthermia probe insertable intoa patient via a normal body opening for providing treatment of saidpatient and said probe having an outer conductive member at least inpart defining an effective heating length of said probe to provide adesired heating pattern, the improvement comprising said outerconductive member formed from a conductor wound in a helical pattern ofaxially varying pitch.
 6. The hyperthermia probe as defined in claim 5further including an inner conductive member extending axially along thelength of said probe.
 7. The hyperthermia probe as defined in claim 6further including a dielectric material disposed about said innerconductive member.
 8. A hyperthermia probe for treatment of a patient,comprising a probe insertable into a normal body opening of the patient,and coupled to a temperature sensing means for detecting temperaturenear said probe, said probe including a conductive inner member and anouter conductive member defining an effective heating length to providea desired heating pattern detectable by said temperature sensing meansand with said outer conductive member comprised of a helical wirewinding comprising an axially varying pitch.